Adhesive sheet for precision electronic member

ABSTRACT

Provided is an adhesive sheet for a precision electronic member which generates less amount of gas under high temperature atmosphere and which is suited as a low polluting adhesive sheet for a precision electronic member. The adhesive sheet for a precision electronic member has an adhesive layer comprising an acrylic copolymer obtained by copolymerizing a monomer mixture which comprises alkyl (meth) acrylate having 4 or less carbon atoms in an alkyl group and vinyl acetate in a proportion of 50:50 to 99.5:0.5 in terms of a weight ratio and in which a total content thereof is 80% by weight or more, wherein an amount of gas generated in heating at a temperature of 120° C. for 10 minutes is 1.0 μg/cm 2  or less in terms of a n-decane amount.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an adhesive sheet for aprecision electronic member. More specifically, the present inventionrelates to an adhesive sheet for a precision electronic member whichgenerates less amount of gas under high temperature atmosphere and whichis suited as a low polluting adhesive sheet for a precision electronicmember.

RELATED ART

[0002] In recent years, an adhesive sheet is used in a variety of thefields because it has such easiness in use that it does not have tochange in a state (for example, from a liquid to a solid) in use as isthe case with conventional adhesives and exhibits an adhesive strengthwhich stands practical use immediately after stuck (pressure-sensitivetack) and because it has such non-pollution, safety and good workabilitythat a solvent and heating are not required in use.

[0003] Capable of being given as uses of this adhesive sheet are, forexample, packaging and binding, business and household, bonding, maskingfor painting, surface protection, corrosion prevention and waterproofing, sealing, electrical insulation, electronic appliances andoptical parts, medical and sanitary material, distinguishing anddecoration and labeling.

[0004] In recent years, particularly in the field of a precisionelectronic member, a diversification in products and a reduction in lotsare advanced, and it has come to be carried out in many cases to stickadhesive labels on which, for example, bar codes are printed ontorespective members for production control or to use an adhesive doublecoated sheet for bonding the members or to use an adhesive sheet forelectric insulation. Such adhesive sheet used for a precision electronicmember is required not to contaminate the above precision electronicmember by gas generated from an adhesive layer. This is because theinside of an electronic appliance equipped with a precision electronicmember is exposed to high temperature atmosphere in use in a certaincase and gas generated from an adhesive sheet stuck onto the precisionelectronic member under such high temperature atmosphere causes bringingabout such undesirable situation as corrosion and malfunction of theabove member.

DISCLOSURE OF THE INVENTION

[0005] The present invention has been made under such circumstances, andan object thereof is to provide an adhesive sheet for a precisionelectronic member which generates less amount of gas under hightemperature atmosphere and which is suited as a low polluting adhesivesheet for a precision electronic member.

[0006] Intensive researches of a low polluting adhesive sheet repeatedby the present inventors have resulted in finding that capable ofmeeting the object described above is an adhesive sheet in which anadhesive comprising an acrylic copolymer having a specific compositionis used for an adhesive layer and in which a gas-generating amountmeasured on a specific condition is controlled to some value or lower.The present invention has been completed based on such knowledge.

[0007] That is, the present invention provides:

[0008] (1) an adhesive sheet for a precision electronic member which hasan adhesive layer comprising an acrylic copolymer and in which an amountof gas generated in heating at a temperature of 120° C. for 10 minutesis 1.0 μg/cm² or less in terms of a n-decane amount, wherein the acryliccopolymer described above is prepared by copolymerizing a monomermixture which comprises alkyl (meth)acrylate having 4 or less carbonatoms in an alkyl group and vinyl acetate in a proportion of 50:50 to99.5:0.5 in terms of a weight ratio and in which a total content thereofis 75% by weight or more,

[0009] (2) the adhesive sheet for a precision electronic member asdescribed in the above item (1), wherein the acrylic copolymer isprepared by copolymerizing alkyl (meth)acrylate having 4 or less carbonatoms in an alkyl group and vinyl acetate with a monomer having across-linkable group, and

[0010] (3) the adhesive sheet for a precision electronic member asdescribed in the above item (1), wherein the weight average molecularweight of the acrylic copolymer is 150,000 or more.

EMBODIMENT OF THE INVENTION

[0011] The adhesive layer in the adhesive sheet of the present inventionis constituted from an adhesive comprising an acrylic copolymer, and theabove acrylic copolymer comprises alkyl (meth)acrylate having 4 or lesscarbon atoms in an alkyl group and vinyl acetate as essential monomers.In this respect, the alkyl (meth)acrylate having 4 or less carbon atomsin an alkyl group includes, for example, methyl (meth)acrylate, ethyl(meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate,n-butyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl(meth)acrylate and t-butyl (meth)acrylate. They may be used alone or incombination of two or more kinds thereof. If the alkyl (meth)acrylatehas 5 or more carbon atoms in an alkyl group, a low polluting adhesivesheet in which a gas-generating amount is controlled to a prescribedvalue or lower is less liable to be obtained.

[0012] The function of vinyl acetate copolymerized with these alkyl(meth)acrylates is not necessarily apparent, and it is considered thatvinyl acetate has a low polymerization reactivity in itself, so that ithas a function to make a polymerization reaction of other monomers toproceed a residual amount of the unreacted other monomers. Even if vinylacetate having a low polymerization reactivity remains as an unreactedmonomer in the adhesive, the above vinyl acetate, which has as lowboiling point as 73° C., is volatilized in heating and drying in formingan adhesive layer and therefore does not exert an adverse effect on thegas-generating amount.

[0013] In the present invention, the monomer mixture which comprisesalkyl (meth)acrylate having 4 or less carbon atoms in an alkyl group andvinyl acetate in a proportion of 50:50 to 99.5:0.5 in terms of a weightratio and in which a total content thereof is 75% by weight or more ispolymerized to produce the acrylic copolymer. If a use amount of vinylacetate is lower than the range described above, the effect ofcopolymerizing vinyl acetate is not exhibited, and the object of thepresent invention can not be achieved. On the other hand, if it exceedsthe range described above, the performance thereof as an adhesive isunsatisfactory. Because of these reasons, a proportion of the abovealkyl (meth)acrylate to vinyl acetate is preferably 70:30 to 99:1,particularly preferably 80:20 to 90:10 in terms of a weight ratio.

[0014] Further, if a total content of alkyl (meth)acrylate having 4 orless carbon atoms in an alkyl group and vinyl acetate in the monomermixture is less than 75% by weight, the performance as an adhesive isunsatisfactory or the gas-generating amount grows larger depending onthe kind of the other monomers, and the desired low polluting adhesivesheet is not obtained in a certain case.

[0015] In the present invention, other copolymerizable monomers can beused, if necessary, together with the alkyl (meth)acrylate and vinylacetate each described above as long as the object of the presentinvention is not damaged. The copolymerizable monomers include, forexample, monomers having cross-linking functional groups or olefins suchas ethylene, propylene and isobutylene; halogenated olefins such asvinyl chloride and vinylidene chloride; styrene monomers such as styreneand α-methylstyrene; diene monomers such as butadiene, isoprene andchloroprene; nitrile monomers such as acrylonitrile andmethacrylonitrile; and N,N-dialkyl-substituted acrylamides such asN,N-dimethylacrylamide and N,N-dimethylmethacrylamide. Among thesecopolymerizable monomers, monomers having cross-linking functionalgroups are preferred, and these monomers having cross-linking functionalgroups include, for example, hydroxyalkyl (meth)acrylates such as2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate and3-hydroxypropyl (meth)acrylate; acrylamides such as acrylamide,methacrylamide, N-methylacrylamide, N-methylmethacrylamide,N-methylolacrylamide and N-methylolmethacrylamide; and ethylenicallyunsaturated carboxylic acids such as acrylic acid, methacrylic acid,crotonic acid, maleic acid, itaconic acid and citraconic acid.

[0016] These copolymerizable monomers may be used alone or incombination of two or more kinds thereof, and a use amount thereof is25% by weight or less based the weight of the whole monomers.

[0017] In the present invention, the acrylic copolymer used for theadhesive is preferably produced by radically polymerizing the monomermixture described above using a conventionally known azo or peroxideradical polymerization initiator. An amount of the radicalpolymerization initiator is suitably 0.01 to 5 parts by weight,preferably 0.1 to 1 part by weight per 100 parts by weight of themonomer mixture. The polymerization form of the acrylic copolymer thusobtained shall not specifically be restricted and may be any of random,block and graft copolymers. The molecular weight is selected in a rangeof preferably 150,000 or more, more preferably 300,000 to 1,500,000 interms of a weight average molecular weight. If this weight averagemolecular weight is less than 150,000, the adhesive is likely to beinferior in a heat resistance and unsatisfactory in an adhesive strengthagainst an adherend.

[0018] The weight average molecular weight described above is a valuereduced to polystyrene measured by gel permeation chromatography (GPC).

[0019] The adhesive in the present invention comprises the acryliccopolymer described above as a resin component, and it further comprisespreferably a cross-linking agent together with this acrylic copolymer.The above cross-linking agent includes, for example, isocyanatecross-linking agents, epoxy cross-linking agents, metal chelatecross-linking agents, aziridine cross-linking agents and amine resins.

[0020] In this respect, the examples of the isocyanate cross-linkingagents include tolylenediisocyanate (TDI), hexamethylenediisocyanate(HMDI), isophoronediisocyanate (IPDI), xylylenediisocyanate (XDI),hydrogenated tolylenediisocyanate., diphenylmethanediisocyanate andtrimethylolpropane-modified TDI. The examples of the epoxy cross-linkingagents include ethylene glycol diglycidyl ether, 1,6-hexanedioldiglycidyl ether, trimethylolpropane diglycidyl ether, diglycidylanilineand diglycidylamine. The examples of the metal chelate cross-linkingagents include chelate compounds comprising acetylacetone and acetoneacid esters of divalent or higher metals such as aluminum, copper, iron,tin, zinc, titanium, nickel, antimony, magnesium, vanadium, chromium andzirconium. The examples of the aziridine cross-linking agents includetrimethylolpropane-tri-β-aziridinylpropionate andbisisophthaloyl-1-(2-methylaziridine).

[0021] These cross-linking agents may be used alone or in combination oftwo or more kinds thereof. A use amount of the cross-linking agent issuitably 0.01 to 20 parts by weight per 100 parts by weight of theacrylic copolymer, and permanent adhesion type and removable typeadhesives can be prepared by controlling a use amount of thecross-linking agent. Further, the adhesive in the present invention canbe blended, if necessary, with various addition components which have sofar conventionally been used for an acrylic adhesive as long as theobject of the present invention is not damaged.

[0022] The adhesive sheet of the present invention has the adhesivelayer thus prepared. The form thereof shall not specifically berestricted, and capable of being used is any of a sheet having anadhesive layer on one face of a base sheet, a sheet having adhesivelayers on both faces of a base sheet, a sheet in which an adhesive layeris interposed between two release sheets without using a base sheet anda sheet in which an adhesive layer is provided on one face of a releasesheet subjected to release treatment on both faces thereof and which isrolled.

[0023] A thickness of the adhesive layer described above shall notspecifically be restricted and is suitably selected according to uses ofthe adhesive sheet. It is selected in a range of usually 5 to 200 μm,preferably 10 to 130 μm. This adhesive layer is preferably formed bycoating the adhesive by a conventionally known method, for example, amethod such as a spin coating method, a spray coating method, a barcoating method, a knife coating method, a roll coating method, a bladecoating method, a die coating method and a gravure coating method andthen heating it at a temperature of 80 to 150° C. for 30 seconds to 5minutes in order to prevent a solvent and low boiling components fromremaining.

[0024] The base sheet described above shall not specifically berestricted, and various ones can be used. To be specific, used arepolyethylene, polypropylene, polyesters such as polyethyleneterephthalate, metal-deposited polyesters, synthetic paper, woodfreepaper, impregnated paper, metallic foils such as aluminum foil, copperfoil and iron foil and porous materials such as nonwoven fabrics. Athickness of these base sheets shall not specifically be restricted, andit is usually 3 to 250 μm and falls in a range of preferably 10 to 200μm in terms of easiness in handling.

[0025] In the adhesive sheet of the present invention, a release sheetcan be provided, if necessary, on the adhesive layer. This release sheetis prepared by using woodfree paper, clay coat paper, woodfree paperlaminated with polyethylene or polypropylene, woodfree paper subjectedto filling treatment with cellulose, starch, polyvinyl alcohol or anacryl-styrene resin or a plastic film of polyethylene, polypropylene orpolyester such as polyethylene terephthalate and subjecting one face orboth faces thereof to releasing treatment. A release agent used for theabove releasing treatment are, for example, olefin resins, a long chainalkyl, a fluorine and a silicone resin. In particular, when the aboveadhesive sheet is used in production process of a hard disc or used as aproduction control label for a hard disc, a silicone release agent istransferred to an adhesive layer in a trace amount, and a siliconcomponent thereof is adhered to a hard disc apparatus; and it forms anoxide thereof on a recording disc and a read head in the hard discapparatus, which results in breaking the above hard disc apparatus in acertain case. Accordingly, when the above adhesive sheet is used foruses in a hard disc, a non-silicone release agent is preferably used.Further, when the silicone release agent is used as a release agent, theagent having less silicone component transferred to the adhesive layeris preferably selected.

[0026] In the adhesive sheet of the present invention, an amount of gasgenerated in heating at a temperature of 120° C. for 10 minutes is 1.0μg/cm² or less in terms of a n-decane amount. If this gas-generatingamount exceeds 1.0 μg/cm², corrosion and malfunction of a precisionelectronic member on which the above adhesive sheet is stuck are broughtabout in a certain case.

[0027] When the above adhesive sheet has a release sheet, thegas-generating amount described above is a value measured after peelingand removing the above release sheet. A specific method for measuringgas generated shall be explained later.

[0028] Capable of being given as a precision electronic member to whichthe adhesive sheet of the present invention is applied are, for example,hard disc apparatuses, semiconductor parts such as an extension memoryand an IC card, semiconductor production apparatuses, relay switches andcircuit boards.

EXAMPLES

[0029] Next, the present invention shall be explained in further detailswith reference to examples, but the present invention shall by no meansbe restricted by these examples.

Example 1

[0030] (1) Preparation of Acrylic Copolymer Solution

[0031] A reactor equipped with a thermometer, a stirrer, a refluxcondenser and a nitrogen gas-introducing tube was charged with 88 partsby weight of butyl acrylate (BA), 2 parts by weight of acrylic acid(AA), 10 parts by weight of vinyl acetate (VA) and 150 parts by weightof ethyl acetate as a solvent, and 0.3 part by weight ofazobisisobutyronitrile was further added thereto as a radicalpolymerization initiator to polymerize them at 80° C. for 8 hours undernitrogen gas atmosphere. After finishing the reaction, toluene was addedto prepare an acrylic copolymer solution having a solid matterconcentration of 32% by weight. The acrylic copolymer thus obtained hada molecular weight of 300,000 in terms of a weight average molecularweight converted to polystyrene which was measured by gel permeationchromatography (GPC) (hereinafter, the same shall apply).

[0032] (2) Production of Adhesive Sheet

[0033] A mixture of low density polyethylene (┌Yukaron LK-30┘,manufactured by Mitsui Chemical Co., Ltd.) and an ethylene-α-olefincopolymer (┌P-0280┘, manufactured by Mitsui Chemical Co., Ltd.) in aweight ratio of 50:50 was laminated as a release layer on one face ofwoodfree paper having a weight of 70 g/m² in a thickness of 30 μm by anextrusion laminate method (melt temperature: 300° C.) to prepare arelease sheet.

[0034] On the other hand, an aluminum chelate cross-linking agent(┌M-5A┘, manufactured by Soken Chemical Co., Ltd.) 1.5 part by weightwas added to 100 parts by weight of the acrylic copolymer solutionobtained in (1) described above to prepare an adhesive.

[0035] Next, the adhesive described above was coated on a back face of apolyethylene terephthalate (PET) film having a thickness of 50 μm(┌Cosmoshine A4100┘, manufactured by Toyo Boseki Co., Ltd.) by a barcoating method using a Mayer bar so that a thickness after drying was 25μm, and it was heated and dried at 110° C. for one minute to form anadhesive layer. Then, the release sheet described above was providedthereon by means of a laminator so that the adhesive layer was broughtinto contact with the release layer to prepare an adhesive sheet.

Example 2

[0036] (1) Preparation of Acrylic Copolymer Solution

[0037] The same reactor as in Example 1 was used and charged with 30parts by weight of ethyl acrylate (EA), 65 parts by weight of butylacrylate, 2 parts by weight of acrylic acid, 3 parts by weight of vinylacetate and 150 parts by weight of ethyl acetate, and 0.3 part by weightof azobisisobutyronitrile was further added thereto to polymerize themat 80° C. for 8 hours under nitrogen gas atmosphere. After finishing thereaction, toluene was added to prepare an acrylic copolymer solutionhaving a solid matter concentration of 34% by weight. The acryliccopolymer thus obtained had a weight average molecular weight of350,000.

[0038] (2) Production of Adhesive Sheet

[0039] An isocyanate cross-linking agent (┌colonate L┘, manufactured byNippon Polyurethane Co., Ltd.) 3 parts by weight was added to 100 partsby weight of the acrylic copolymer solution obtained in (1) describedabove to prepare an adhesive.

[0040] An adhesive sheet was prepared in the same manner as in Example 1(2), except that this adhesive was used.

Example 3

[0041] (1) Preparation of Acrylic Copolymer Solution

[0042] The same reactor as in Example 1 was used and charged with 20parts by weight of ethyl acrylate, 48 parts by weight of butyl acrylate,2 parts by weight of acrylic acid, 30 parts by weight of vinyl acetateand 150 parts by weight of ethyl acetate, and 0.3 part by weight ofbenzoyl peroxide was further added thereto as a radical polymerizationinitiator to polymerize them at 80° C. for 8 hours under nitrogen gasatmosphere. After finishing the reaction, toluene was added to preparean acrylic copolymer solution having a solid matter concentration of 34%by weight. The acrylic copolymer thus obtained had a weight averagemolecular weight of 350,000.

[0043] (2) Production of Adhesive Sheet

[0044] The isocyanate cross-linking agent (┌Colonate L┘, manufactured byNippon Polyurethane Co., Ltd.) 3 parts by weight was added to 100 partsby weight of the acrylic copolymer solution obtained in (1) describedabove to prepare an adhesive.

[0045] An adhesive sheet was prepared in the same manner as in Example 1(2), except that this adhesive was used and coated so that a thicknessafter drying was 50 μm.

Comparative Example 1

[0046] (1) Preparation of Acrylic Copolymer Solution

[0047] The same reactor as in Example 1 was used and charged with 88parts by weight of 2-ethylhexyl acrylate (2EHA), 2 parts by weight ofacrylic acid, 10 parts by weight of vinyl acetate and 150 parts byweight of ethyl acetate, and 0.3 part by weight ofazobisisobutyronitrile was further added thereto to polymerize them at80° C. for 8 hours under nitrogen gas atmosphere. After finishing thereaction, toluene was added to prepare an acrylic copolymer solutionhaving a solid matter concentration of 36% by weight. The acryliccopolymer thus obtained had a weight average molecular weight of370,000.

[0048] (2) Production of Adhesive Sheet

[0049] The isocyanate cross-linking agent (┌Colonate L┘, manufactured byNippon Polyurethane Co., Ltd.) 3 parts by weight was added to 100 partsby weight of the acrylic copolymer solution obtained in (1) describedabove to prepare an adhesive.

[0050] An adhesive sheet was prepared in the same manner as in Example 1(2), except that this adhesive was used.

Comparative Example 2

[0051] (1) Preparation of Acrylic Copolymer Solution

[0052] The same reactor as in Example 1 was used and charged with 58parts by weight of 2-ethylhexyl acrylate, 30 parts by weight of butylacrylate, 2 parts by weight of acrylic acid, 10 parts by weight of vinylacetate and 150 parts by weight of ethyl acetate, and 0.3 part by weightof azobisisobutyronitrile was further added thereto to polymerize themat 80° C. for 8 hours under nitrogen gas atmosphere. After finishing thereaction, toluene was added to prepare an acrylic copolymer solutionhaving a solid matter concentration of 32% by weight. The acryliccopolymer thus obtained had a weight average molecular weight of330,000.

[0053] (2) Production of Adhesive Sheet

[0054] The isocyanate cross-linking agent (┌Colonate L┘, manufactured byNippon Polyurethane Co., Ltd.) 3 parts by weight was added to 100 partsby weight of the acrylic copolymer solution obtained in (1) describedabove to prepare an adhesive.

[0055] An adhesive sheet was prepared in the same manner as in Example 1(2), except that this adhesive was used.

[0056] The respective adhesive sheets obtained in Examples 1 to 3 andComparative Examples 1 and 2 were evaluated for an adhesive strength, agas-generating amount and an effect to an electronic member according tomethods shown below. The results thereof are shown in Table 1.

Evaluation of Adhesive Sheet

[0057] (1) Adhesive Strength

[0058] An adhesive strength to a cold rolled stainless steel plate wasmeasured according to JIS Z0237.

[0059] (2) Gas-generating Amount

[0060] The adhesive sheet from which the release sheet was peeled andremoved was heated at 120° C. for 10 minutes in a purge & trap(┌JHS-100Al┘, manufactured by Nippon Electron Ind. Co., Ltd.) to samplegas, and it was then introduced into GC Mass (┌Turbo Mass┘, manufacturedby Perkin Elmer Co., Ltd.) to determine an amount of generated gas interms of a n-decane amount. The amount in terms of a n-decane amount wasdetermined from a calibration curve of n-decane prepared in advance bysetting a detection intensity of generated gas obtained by GC Mass as adetection intensity of n-decane.

[0061] (3) Effect to Electronic Member

[0062] The adhesive sheet was stuck on an extension memory for apersonal computer and left standing while heating, and then it wasconfirmed if the memory was recognized on a personal computer.

[0063] The adhesive sheet of 20 mm×115 mm was stuck on an extensionmemory (for NXJ-E256MB, manufactured by Melco Co., Ltd.), and this wasput in a sealed glass vessel and left standing at 80° C. for 168 hours.Then, the memory was connected to a personal computer (┌PC-98 Mate MXMA23D┘, manufactured by NEC Co., Ltd.) to confirm if the memory wasoperated. TABLE 1 Comparative Example Example 1 2 3 1 2 AcrylicCopolymer Monomer use amount (parts by weight) BA 88 65 48 — 30 EH — 3020 — — 2EHA — — — 88 58 AA 2 2 2 2 2 VAc 10 3 30 10 10 Weight average300,000 350,000 350,000 370,000 330,000 molecular weight Adhesive sheetAdhesive strength 10.4 11.8 20.1 9.3 12.5 (N/25 mm) Gas-generating 0.180.31 0.89 1.43 1.20 amount (μg/cm²) Effect to electronic Good Good GoodNot Instable member opera- opera- opera- recognized opera- tion tiontion tion

[0064] In Comparative Example 1, 2-ethylhexanol (boiling point: 184° C.)originating in 2-ethylhexyl acrylate was detected, and a large amount ofgas is generated. Accordingly, an effect to an electronic member isobserved.

[0065] In Comparative Example 2, butyl acrylate is used for a part ofthe main component for the acrylic copolymer, so that less amount of gasthan in Comparative Example 1 is generated, but an effect to anoperability of an electronic member is observed.

EFFECTS OF THE INVENTION

[0066] According to the present invention, capable of being readilyobtained is an adhesive sheet for a precision electronic member whichgenerates less amount of gas under high temperature atmosphere and whichis suited as a low polluting adhesive sheet for a precision electronicmember.

What is claimed is:
 1. An adhesive sheet for a precision electronicmember which has an adhesive layer comprising an acrylic copolymer andin which an amount of gas generated in heating at a temperature of 120°C. for 10 minutes is 1.0 μg/cm² or less in terms of a n-decane amount,wherein the acrylic copolymer described above is prepared bycopolymerizing a monomer mixture which comprises alkyl (meth)acrylatehaving 4 or less carbon atoms in an alkyl group and vinyl acetate in aproportion of 50:50 to 99.5:0.5 in terms of a weight ratio and in whicha total content thereof is 75% by weight or more.
 2. The adhesive sheetfor a precision electronic member as described in claim 1, wherein theacrylic copolymer is prepared by radically copolymerizing alkyl(meth)acrylate having 4 or less carbon atoms in an alkyl group, vinylacetate and a monomer having a cross-linkable group.
 3. The adhesivesheet for a precision electronic member as described in claim 1, whereinthe weight average molecular weight of the acrylic copolymer is 150,000or more.